Enter Anna Chapman and The Illegals Program

While he was languishing in a Russian cell, events were underway in the USA which were to see Skripal gain his eventual freedom.

A network of Russian sleeper agents dubbed The Illegals Program by US authorities were posing as ordinary Americans and trying to elicit useful information from policymakers, analysts and business people.

The 10 agents were arrested in 2010 after a decade-long FBI operation.

A new life in England

As part of the trade deal, Skripal was granted asylum in the UK where he had kept a low profile for the past eight years.

He had been living openly in Britain under his own name, and his address in Salisbury appeared in public records.

On Sunday, he was found slumped unconscious on a bench at a Salisbury shopping centre alongside his daughter Yulia, who was reportedly visiting him from Moscow.

The pair are fighting for their lives in hospital, while a member of the emergency services who treated them is also unwell.

UK Foreign Secretary Boris Johnson has warned Russia is a "malign and disruptive force in the world" and has warned foreign governments that "no attempt to take innocent life on UK soil will go either unsanctioned or unpunished".

A British inquest concluded that Litvinenko had been murdered by Russian agents with the probable approval of Russian President Vladimir Putin.

In a sign of how seriously the British are taking this, Mr Johnson issued a veiled threat to pull England out of the football World Cup — whish is being held later this year in, you guessed it, Russia.

Mr Johnson also pointed to further sanctions against Russia, at a time when relations are already described as being at their lowest point since the Cold War.

The 'poisoned pint' theory

The investigation has entered its fourth day and Britons have woken up to front pages splashed with new theories on what happened to Skripal.

The Daily Telegraph claims Vladimir Putin always wanted Sergei Skripal killed and had "sworn revenge" on him after he was sent to the UK.

The Sun reports that a "Kremlin assassin" may have spiked Skripal's pint of beer with a "rare and almost untraceable rat poison".

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